The fabrication and assembly of most apparatus, equipment and devices entail the fastening or fixing of various parts together for securing the same as a unit. In many cases, various fastening techniques are available, one or more of which may be selected because of expediency, cost or availability. However, in other instances, the fastening together of elements can be reliably achieved only after numerous critical factors have been considered, such as the types of materials, the thermal expansion characteristic of the materials, the temperatures and ranges which the devices will experience, the adhering or bonding technique, vibration, long term stability, electrical conductivity and other concerns.
Transducers comprise a type of apparatus which involves numerous assembly considerations due to the concerns noted above. Particularly, balanced differential pressure transducers, of the capacitance displacement type, are fabricated with a sandwiched glass and silicon structure with metallized conductive surfaces, the assembly of which is packaged in a metal case. Pressure conduits must be provided for allowing pressure differentials to act upon a transducer silicon diaphragm. Also, electrical conductors must be provided external to the case for sensing capacitance changes of the transducer diaphragm. In addition, the transducer wafer structure itself must be supported within the case in such a manner as to allow the noted pressure and electrical connections thereto, as well as a support for securing the transducer structure within the case. Because such type of devices are typically utilized in an environment in which broad temperature excursions may be experienced, and in which the device may be required to withstand substantial vibrations, the assembly of such a structure can present numerous technical problems which must be overcome to assure a highly reliable device.
Some of the considerations and concerns which must be addressed during the assembly of a multi-element structure, such as the foregoing, are the thermal expansions properties of different materials with which, if not considered, can crack or break the transducer-enclosure interface connections and destroy the device. Also, and especially in capacitive sensitive devices, the type of interface and bonding of parts together must be considered so that stray capacitance is minimized, and does not change over the life of the transducer.
Recent developments in the polymer area have given rise to the availability of high temperature polyimide resins which exhibit excellent bonding properties. Polyimide film laminates have been employed in space vehicles with a great deal of success. Polyimide naterials and bonding techniques are disclosed in U.S. Pat. Nos. 4,065,345, 4,094,862 and 4,543,295. The disclosures of these patents, which are incorporated herein by reference, relate to the development of the LARC-TPI bonding technology by the National Aeronautics and Space Administration.
Conventional processing techniques for bonding or adhering surfaces together with polyimide resins include the steps of spreading a thin layer of the liquid polyimide material on a film or solid surface, and heating the coated material to a high temperature to drive off the solvent and completely imidize the polyimide material. The initial heating step drives off all the solvent in the polyimide adhesive so that bubbles do not form during subsequent processing steps. Curing of the polyimide is accomplished by continuing to increase its temperature, forming imide rings along the polymer structure, thus yielding good mechanical properties and still being thermoformable. After complete imidizing of the polyimide adhesive, another material or laminate layer is pressed onto the imidized polyimide adhesive. The laminate, structure is then heated under pressure to form a thermoplastic bond. It can be appreciated that if voids, bubbles or blisters form in the polyimide adhesive, an incomplete or nonuniform bonding interface is formed which is more easily stressed or broken.
From the foregoing, it can be seen that a need exists for an improved technique to assemble and fix a transducer wafer assembly within a sealed enclosure to provide a cost effective and reliable device. Another need exists for a technique to bond or adhere a glass part of the transducer wafer assembly to a support structure such that temperature, vibration and other affects are minimized.